Backside gettering of impurities in silicon semiconductors is a common practice. For example, U.S. Pat. No. 4,053,335, issued on Oct. 11, 1977, describes the depositing of polycrystalline silicon, hereinafter polysilicon, on the backside of the silicon wafer. Thereafter, during high temperature processing, impurities are alleged to migrate into the polysilicon and become trapped in the polycrystalline grain boundaries.
There is little question that, once the impurities enter the polysilicon layer, they will be trapped by the grain boundaries. However, in practice it has been found that the polysilicon grain boundaries noted in U.S. Pat. No. 4,053,335, col. 3, lines 27-29 inhibit intimate molecular contact and thus prevent the impurities from migrating to the polysilicon in the first place. This condition is worsened if any silicon oxide is present, since the silicon oxides on the wafer further interfere with transfer of impurities to the polysilicon layer. Thus, the process described in U.S. Pat. No. 4,053,335 has not been optimally effective, so that the art has had to turn to other approaches.
For example, U.S. Pat. No. 4,131,487, issued on Dec. 26, 1978, describes a technique for backside gettering wherein crystal defects are introduced into a single crystal by scanning portions of it with a laser powered sufficiently to physically damage the crystal, and heating the device to cause the damage or defects to form dislocations. These dislocations act as gettering means or traps for impurities. Such a technique is disadvantageous because, as described in U.S. Pat. No. 4,257,827, issued on Mar. 24, 1981, unless the dislocation formation is done with care, the dislocations migrate to the front of the wafer and interfere with device performance. The alternative taught by U.S. Pat. No. 4,257,827 is to use a lower power laser that melts the backside of the wafer, and "activates" oxygen complexes as gettering centers. This process is disadvantageous in that (a) not all impurities are readily gettered by such a technique, and (b) minority carrier lifetimes decrease in the interior of the wafer due to damage to the single crystalline wafer from the incident laser beam and to formation of the oxygen complex gettering centers.
Thus, prior to the instant invention, conventional gettering techniques have suffered from serious drawbacks.